Unidirectional vs non-unidirectional airflow

Cleanrooms control airborne particles in one of two ways. They either sweep particles away from the work with a uniform, parallel flow of clean air, or they dilute particles by mixing large volumes of clean air into the room. ISO 14644 calls the first unidirectional airflow and the second non-unidirectional airflow. Many facilities still call them laminar and turbulent flow, although true laminar flow in the fluid dynamics sense rarely exists in a cleanroom.

The choice between them sets the cleanliness a zone can achieve, its sensitivity to disturbance and its energy use. Most real facilities use both, with unidirectional flow only where it is essential.

How unidirectional airflow works

In a unidirectional zone, filtered air enters across the whole ceiling (vertical flow) or a whole wall (horizontal flow) at a uniform velocity. It moves in roughly parallel streamlines through the work area and leaves through a perforated floor, low wall returns or the opposite wall. Particles released in the zone are carried downstream and out before they can spread sideways.

Key design features:

• Full filter coverage. HEPA or ULPA filters cover most of the ceiling or wall, typically with filter coverage well above half the area and often close to 100 percent for the cleanest zones.
• Uniform velocity. Velocity is typically in the range of about 0.2 to 0.5 m/s (40 to 100 fpm). EU GMP Annex 1 gives a guidance range of 0.36 to 0.54 m/s at the working position for Grade A. Uniformity is commonly specified within plus or minus 20 percent of the average.
• Unobstructed path. Equipment, work surfaces and people disturb the flow. Perforated work surfaces, streamlined equipment and keeping hands and objects downstream of critical points help.
• Low-level or floor returns. Return air must leave on the far side of the work so streamlines stay parallel.

Where unidirectional airflow is used

• GMP Grade A zones for aseptic processing.
• Semiconductor lithography and other ISO 3 to ISO 5 areas.
• Biological safety cabinets, laminar flow hoods and isolators.
• Hospital pharmacy primary engineering controls (ISO 5).

Instead of making an entire room unidirectional, designers increasingly use small local devices such as clean benches, minienvironments, RABS or isolators. These provide unidirectional ISO 5 conditions exactly where the product is exposed, while the surrounding room uses non-unidirectional airflow.

How non-unidirectional airflow works

In a non-unidirectional room, clean air enters through HEPA filters at discrete locations in the ceiling, mixes with room air and leaves through returns. Particles are diluted rather than swept away. The cleanliness achieved depends on the air change rate, the particle generation rate and how well the air mixes.

Key design features:

• Distributed supply. Terminal HEPA filters or fan filter units spread evenly across the ceiling, often with diffuser faces that promote mixing.
• Low-level returns. Returns near the floor at the perimeter draw air downward past the work and away from surfaces. Ceiling returns can short-circuit supply air and should be avoided in critical rooms.
• Air change rate set by calculation. See air change rates and airflow in cleanrooms.

Non-unidirectional airflow typically serves ISO 6 to ISO 8 rooms and GMP Grade B, C and D backgrounds.

Side-by-side comparison

Mixed-flow and hybrid designs

Some rooms combine approaches. A common example is a non-unidirectional ISO 7 room with a unidirectional canopy over a critical process line. The canopy's air eventually mixes with the rest of the room. Its downflow must reach the work surface with enough momentum to resist room air currents, and the boundary between zones should be defined by curtains, partial walls or physical barriers where possible.

Another approach is to locate a cluster of fan filter units over a work area within a larger room, sometimes called a clean zone. This can achieve a better class locally but is sensitive to people walking through and to room air currents, so performance must be verified under real working conditions.

Common problems

Unidirectional zones

• Obstructions create wakes downstream where particles accumulate or recirculate upward.
• Thermal plumes from hot equipment rise against downflow and disturb streamlines.
• Non-uniform velocity at the filter face, often from poor plenum design or uneven fan filter unit speeds, causes eddies.
• Entrainment at the edges of the zone draws dirtier room air into the clean zone if the boundary is not defined.
• Incorrect return paths turn vertical flow sideways before it leaves the work area.

Non-unidirectional rooms

• Dead zones in corners or behind equipment where air does not reach.
• Short-circuiting of supply air to nearby returns.
• Equipment blocking returns, a common problem once a room is fitted out.
• Doors and personnel movement creating local disturbances.

Verifying airflow performance

1. Measure airflow velocity and uniformity in unidirectional zones using methods in ISO 14644-3, usually with a grid of measurement points just below the filter face or at the working position.
2. Measure total airflow and calculate air change rates in non-unidirectional rooms.
3. Perform airflow visualization (smoke studies). Visible smoke or fog shows the actual flow pattern. In GMP environments, dynamic smoke studies are performed with operators carrying out simulated interventions and are video recorded.
4. Check particle counts at critical locations under operational conditions.
5. Perform recovery tests to confirm how quickly the room clears after contamination.

Smoke studies often reveal issues that measurements miss, such as an operator's arm creating turbulence directly over an open container. Results feed back into equipment layout and procedures.

Choosing the right approach

1. Identify where product or critical surfaces are exposed and what class is needed there.
2. Provide unidirectional airflow only for those zones, using the smallest practical device.
3. Specify the background room class needed to support that device, as discussed in GMP cleanroom grades A to D.
4. Design background rooms with non-unidirectional airflow, distributed supply and low returns.
5. Choose supply equipment, comparing options in fan filter units vs ducted HEPA systems.
6. Plan verification, including smoke studies, from the start.

Key takeaways

• Unidirectional airflow sweeps particles away with uniform parallel flow; non-unidirectional airflow dilutes them by mixing.
• Unidirectional zones are designed by velocity and uniformity, typically about 0.2 to 0.5 m/s, with 0.36 to 0.54 m/s guidance for GMP Grade A.
• Non-unidirectional rooms rely on distributed supply, low-level returns and a calculated air change rate.
• Limiting unidirectional flow to local devices around exposed product saves large amounts of energy.
• Smoke studies under real working conditions are the best way to confirm airflow patterns.